Generally speaking, identification symbol scanning entails the aiming of an image acquisition sensor (CMOS camera, CCD, etc.) at a location (a “region of interest”) on an object that contains a bar code or other decodable indicia and retrieval of an image of that symbol. The decodable indicium contains a set of predetermined patterns that represent an ordered group of characters or symbols from which a processor can, for example, derive useful information about the object (e.g. its serial number, type, model, price, etc.).
In reading decodable indicia, the type of illumination employed is of particular concern. Where decodable indicia are printed on a flat surface with contrasting ink or paint, a diffuse, high-angle “bright field” illumination may best highlight these features for the sensor. By high-angle it is meant, generally, light that strikes the subject nearly perpendicularly (normal) or at an angle that is typically no less than about 45 degrees from perpendicular (normal) to the surface of the item being scanned. Such illumination is subject to substantial reflection back toward the sensor. By way of example, decodable indicia requiring mainly bright field illumination may be present on a printed label adhered to an item or container, or on a printed field in a relatively smooth area of an item or container. Some direct part marking scanners use a dome or cone-shaped diffuser to provide the totally diffuse bright field illumination for specular surfaces.
Where a decodable indicium (e.g., a barcode) is formed on a more-irregular surface or is created by etching or peening a pattern directly on the surface (i.e., an example of direct part marking), the use of highly reflective bright field illumination may be inappropriate. A peened/etched surface has two-dimensional properties that tend to scatter bright field illumination, thereby obscuring the acquired image. Where an etched or peened surface has such decidedly two-dimensional surface texture, it may be best illuminated with dark field illumination. This is an illumination with a characteristic low angle (approximately 45 degrees or less, for example) with respect to the surface of the subject (i.e. an angle of more than approximately 45 degrees with respect to normal). Using such low-angle, dark field illumination, two-dimensional surface texture is contrasted more effectively (with indents appearing as bright spots and the surroundings as shadow) for better image acquisition. To take full advantage of the versatility of a camera image sensor, it is desirable to provide both bright field and dark field illumination for selective or simultaneous illumination of a subject.
However, dark field illumination must be presented close to an object to attain the low incidence angle thereto. Conversely, bright field illumination is better produced at a relative distance to ensure full area illumination. Light pipes in the form of a light ring surrounding the front edge of the diffuser have been used to produce low angle dark field illumination when the light pipe is touching or near touching the object on which the decodable indicium is etched or peened. Light pipe illumination is acceptable with highly reflective or specular reflective surfaces with low contrast marking. However, for low reflective or diffused reflective surfaces, light pipes cannot provide sufficient contrast.
Therefore, a need exists for direct part marking scanners including dome diffusers with edge illumination assemblies. Various embodiments provide dark field illumination at contact and near contact distance as well as near and far field bright field illumination (i.e., contact and near contact dark field and bright field illumination as well as mid- and far-distance bright field illumination). Various embodiments also provide high or near normal incidence angle illumination for the object at a distance from a front edge of the diffuser while providing low incidence angle illumination for the object at contact or near contact distance.